The concept of the home deployed base station, or femtocell, is considered of increasing importance for cellular network operators. Femtocells operate at low downlink transmit power, and are designed to improve the cellular coverage within a home or enterprise environment and their immediate surroundings. Typically a femtocell would be linked into the wider cellular Radio Access Network through a customer's broadband link (e.g. digital subscriber line, cable, passive optical network or other wireline access technology), and provide user equipment terminals with access to data.
The term “base station” is used here to refer to a radio transceiver connected to a telecommunications network; a cell site may have several base stations, each serving a different area of wireless coverage. This deployment of multiple base stations at a cell site is particularly common for macrocellular networks, whereas typically femtocell base stations are intended to be deployed individually, and accordingly are equipped with an omni-directional antenna. The user equipment terminal, often a mobile device such as a smart phone, Personal Digital Assistant (PDA) or laptop and the like, is alternatively referred to as a “user equipment”.
The use of femtocells is particularly applicable in high capacity packet data cellular wireless communication systems such as HSPA (‘High Speed Packet Access’), a so-called third generation evolutionary system, and LTE (Long Term Evolution), often referred to as a fourth generation (4G) system. Services using such systems can typically accommodate a variable data rate to and from the user equipment, and can exploit a greater data rate should it be available, for example for the faster transfer of data files. It is accordingly advantageous to maximise the data capacity available to a user, and to this end adaptive modulation and coding is typically employed. The provision of a femtocell within a subscriber's premises can provide a high capacity link within a small local area that will typically also be within the coverage area of a macrocell.
Although generally placed indoors, femtocells operate within an existing conventional cellular wireless network, which is termed a macrocellular network. There may typically be hundreds of femtocells for every macrocell. The large number of femtocells may interfere with the signal from the macrocells, particularly in the downlink direction from the macrocell base station to the user equipment, in some cases preventing access altogether. This problem is accentuated in the case of “closed access” femtocells which can only be used by a limited group of user equipments. User equipments outside the closed access group may receive a strong signal from the femtocell, however as they cannot use it, it acts as interference to signals received from macrocells.
FIG. 1 illustrates the problem of interference in the closed access case. A femtocell base station 12 is in communication with a user equipment 16. Nearby, a second user equipment 14 receives a strong signal from the femtocell 12 but cannot establish a connection with the femtocell, as the femtocell is closed to access by the second user equipment 14. As mentioned above, the signal from the femtocell base station 12 conventionally occupies the same frequency band as is occupied by the macrocell base station 10, so that the signal from the femtocell base station 12 potentially acts as interference with signals received from macrocell base station 10 at a second user equipment 14.
A second problem relating to the provision of a large number of femtocells within the coverage area of a macrocell base station is the expenditure of power by a user equipment when performing measurements for handover decisions: it can be expected that there is a larger number of near neighbours than are present in a conventional macrocell system, and this will trigger a commensurately larger number of handover-related actions on the part of the user equipment than is experienced in macrocell systems. These actions involve processing on the part of the user equipment, which is particularly undesirable given that the battery life of a user equipment, typically a handset, should be maximised.
One known solution to these problems is for femtocells to use a different frequency channel to that used by a macrocell deployed in the same area. Whilst this avoids interference problems, it is undesirable to operators, since spectrum is expensive to acquire.
Another known alternative is to use an interference mitigation technique: femtocells base stations detect the level of interference they are causing to the user equipments served by a macrocell base station operating in the same area, and reduce their power accordingly. However, this will potentially limit the coverage area and data rate available to users of the femtocell.
It is an object of the present invention to provide a method and apparatus which addresses these disadvantages.